The first time in roughly four decades that a privately developed, non‑light‑water advanced reactor has gone critical in the U.S.
Posted by Leslie Eastman
The quiet revolution in American energy just cleared a major technical hurdle.
At Idaho National Laboratory, Antares Nuclear’s Mark-0 microreactor achieved zero-power criticality under the Department of Energy (DOE) Reactor Pilot Program (RPP), becoming the first advanced reactor to meet the ambitious July 4, 2026, deadline set by President Trump’s 2025 executive order to accelerate next-generation nuclear deployment.
The U.S. energy secretary called the development “one of the most significant achievements in nuclear energy in forty years.”
The Department of Energy said the Mark-Zero reactor design by Antares Nuclear, hosted at Idaho National Laboratory, achieved criticality. That means the team started a self-sustaining chain reaction of nuclear fission, the fundamental process for operating a reactor.
The Department of Energy said it is the first time a privately developed, non-light-water reactor has gone critical in four decades, and that it serves as a blueprint for other advanced reactors supported by the Trump administration.
State leaders praised the accomplishment. Sen. Jim Risch posted on social media that Idaho is “leading the charge” to a new era of advanced reactors.
WE HAVE CRITICALITY! On June 4, 2026, at around 12:30 MDT, @AntaresNuclear’s Mark-0 microreactor achieved initial criticality at @INL.
It is the first nuclear test reactor to go critical under @ENERGY’s Reactor Pilot Program. Congratulations to Antares on reaching this… pic.twitter.com/JS9g9Vsm26
— Office of Nuclear Energy | US Department of Energy (@GovNuclear) June 4, 2026
The zero‑power test was designed to confirm that the core’s reactivity, control systems, and instrumentation behave as predicted, before any attempt at higher‑power operation. The next step in the plan is to get the system to generate sustained electricity, and that phase is also scheduled for this year.
“Now that Mark-0 is critical, the real work is just beginning,” said Antares CEO Jordan Bramble in a LinkedIn post on June 4. “I want to reiterate how this fits into our larger roadmap to mature our technology to its commercial potential. This should be obvious, but the goal of a reactor is to sell electricity to customers.”
Following reactor physics experiments, Antares will execute “the next phase of our roadmap—sustained electricity production,” Bramble said. Antares is “able to move fast towards this milestone because we’ve already completed over 6 months of full-power thermal testing in an electrical prototype. We will perform version 2.0 of this in 2026. This is an easier, more iterative way to test, because there is no regulatory process, and you can disassemble to examine material effects.”
He added: “All of our iterative testing sets us up to produce electricity for 6+ months. Hundreds of days, not hundreds of hours. We’re able to test for longer and faster because we’ve designed our reactor around a proven, fully qualified fuel spec developed under Project Pele.”
Some of the great Americans who made Mark-0 possible. Including @Waksman84 @colejust12 @JessGehin @OverReactor1776
Jeff’s work on project Pele began 8 years ago. The program trained operators, improved the regulatory process, and created the fuel line that made our TRISO. pic.twitter.com/cjpJcGqrKN
— Jordan Bramble (@jordanbramble) June 6, 2026
Presidential Executive Order 14301, “Reforming Nuclear Reactor Testing at the Department of Energy,” the RPP also supports the Army’s Janus Program, which focuses on deploying advanced nuclear microreactors for operational and installation energy needs.
The Janus Program is the Army’s answer to Presidential Executive Order 14299, “Deploying Advanced Nuclear Reactor Technologies for National Security.” The Army coordinated with the Energy Department to ensure fuel was fabricated in time to support the criticality timeline and also provided expertise to support the safety review of the Antares criticality test.
“This is a historic moment for advanced nuclear energy in the United States,” Energy Secretary Chris Wright said. “The Antares team has achieved a major milestone in the development of micro-reactor technology, and the Trump administration is proud to have supported their efforts. These technologies will play a critical role in strengthening U.S. energy security for both military and civilian applications.”
Jeff Waksman, principal deputy assistant secretary of the Army for installations, energy and environment, said, “The criticality test of Antares Nuclear’s Mark-0 reactor is an important step toward meeting the president’s goals for nuclear power resiliency for the U.S. Army. A microreactor is now generating neutrons.”
When America sets bold goals, America delivers.
Last year, President Trump challenged our nation to bring multiple advanced reactors to criticality by America’s 250th Anniversary. Today, we reach an important milestone as the first non-light-water reactor goes critical in more… https://t.co/XWuuEF92Dy
— Secretary Chris Wright (@SecretaryWright) June 4, 2026
This milestone, while not yet producing electricity, confirms the reactor’s core physics and safety behavior in the real world, not just in modeling, and sets the stage for operational microreactors expected as early as 2027.
In an energy landscape increasingly defined by grid instability and rising demand, that’s not just progress, but a signal that U.S. nuclear innovation may finally be moving at the speed policymakers have been demanding.
Why do we keep hearing about this person. What exactly has he done other than mislead, bullied and lied.
The person called mann.
Posted comments in wrong article.
I bet a lot of the AI promoters are paying a lot of attention to these nuclear developments.
These are game changers.
The world may look a whole lot different in a few years.
Progress. Cool. I am rather shocked the nuclear power naysayers aren’t here braying their bullshyte.
Spoke too soon.
Actually right on time. Took them quite a few hours to start braying.
Nowhere in the above article is there a reference to the projected overall power conversion efficiency of the Antares Nuclear’s Mark-0 microreactor. Achieving nuclear fission criticality in a prototype, subscale test is one thing, but in terms of technology usefulness of new nuclear reactor designs, power efficiency is far more important.
And then there is this:
“Jeff Waksman, principal deputy assistant secretary of the Army for installations, energy and environment, said, ‘The criticality test of Antares Nuclear’s Mark-0 reactor is an important step toward meeting the president’s goals for nuclear power resiliency for the U.S. Army. A microreactor is now generating neutrons’.”
(my bold emphasis added)
Well, one can today purchase portable, nuclear FUSION devices—ones based on inertial electrostatic confinement (IEC) fusion—that produce copious neutrons. Primary commercial suppliers for these systems include:
— Starfire Industries (https://www.starfireindustries.com/ngen )and Adelphi Technology (https://www.adelphitech.com/ ) in the United States,
— NSD Gradel Fusion in Europe (http://www.nsd-fusion.com/ ).
However, I do make allowance for the fact that Mr. Waksman is just a deputy assistant secretary in the US Army.
You missed the point.
This is the first system that generated enough energy to sustain the reaction.
Errrr . . . what about the graphite block reactor named Chicago Pile-1 (CP-1), located under the stands of Stagg Field (a squash court) at the University of Chicago, that achieved criticality and the first manmade sustained nuclear fission reaction on December 2, 1942?
And what about the Mark 1 Prototype Naval Reactor (also known as the Submarine Thermal Reactor, or STR), The first pressurized water reactor (PWR) to achieve criticality, which started up in Idaho on March 30, 1953. It was built to test the feasibility of a nuclear reactor for submarines.
And what about the first sodium-cooled reactor to achieve criticality, the Experimental Breeder Reactor I (EBR-I), developed by Argonne National Laboratory. It first went critical on December 20, 1951, at the Idaho National Laboratory site.
And what about the first pebble bed nuclear reactor to achieve criticality, the Arbeitsgemeinschaft Versuchsreaktor (AVR) located at the Jülich Research Centre in Germany. It successfully achieved its first criticality on August 26, 1966.
Abd what about the Akademik Lomonosov in Russia—a floating nuclear power plant utilizing two KLT-40S reactors—which was the first Small Modular Reactor (SMR) to achieve criticality, reaching this milestone on November 2, 2018. It was fully commissioned and connected to the grid by May 2020.
The point, of course, being that facts matter.
Let’s see now: 1942, 1951, 1953 ….. 2026 – in the US. I wonder why there’s such a gap.
It’s the first of a new breed of reactors and it’s the first new reactor in decades.
So funny a comment . . . didn’t you notice this statement in the above article:
“The Department of Energy said it is the first time a privately developed, non-light-water reactor has gone critical in four decades . . .”
The world’s first heavy water nuclear reactor, Chicago Pile-3 (CP-3), was successfully tested and achieved its first sustained nuclear chain reaction on May 15, 1944.
Let’s see . . . that was some 82 years ago . . . so, yeah, not a “new breed” of reactor.
Did you instead mean to say it was a new size of reactor that achieved criticality, since it is asserted to be a “microreactor”?
I misread the article.
I thought it was about a fusion reactor.
I apologize for that error.
It is still a remarkable advancement.
Next up after fusion is the matter anti-matter reactor.
🙂
Ok. Then we can look forward to Mr. Fusion.
🙂
Apparently someone (down vote) has no sense of humor.
Nowhere does it say this is a fusion reactor which is still where it has for my 76 years, still 30 years in the future. Why are they hiding the actual fission technology that is uses. Only that it is not a lightwater reactor.
The first time in roughly four decades that a privately developed, non‑light‑water advanced reactor has gone critical in the U.S.
light water or non-light water, both mean it’s fission
gone critical also means it’s fission.
“That means the team started a self-sustaining chain reaction of nuclear fission, the fundamental process for operating a reactor.”
Ummmmm . . . the title of the above article mentions “criticality” and the article’s lead-in sentence mentions the “reactor has gone critical”.
The terms “criticality” and “critical” are used when referring to sustained nuclear fission reactions, not to nuclear fusion reactions that instead refer to achieving “break even”.
“They” are not hiding anything, but are instead relying on a reader possessing some basic knowledge.
Okay someone help me out. Is this the same as a Small Modular Reactor? If not how are they different? I thought we were already building SMRs
SMRs are scaled for larger electricity output.
Micro-reactors are for local applications, military bases and the like.
SMR require periodic refuelling.
Micro-reactors, as reported, do not get refueled during their operational life.
SMRs have abundant safety features.
Micro-reactors, as reported, are no susceptible to meltdowns.
SMRs are assembled onsite (major components factory produced).
Mirco-reactors, as reported, are delivered fully assembled.
Basically, they address different markets.